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    <title>Document</title>
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        /**
         *document是对整个网页文档的抽象 
         * document。getElementById（“con”）
         * 表示再整个网页文档中根据元素的id属性值获取到
         * 该元素的DOM节点对象
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        var con = document.getElementById("con");
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        img1.onclick = function(){
            /*在事件的回调函数中，this指向当前触发事件的
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            */
            this.style.border = "1px solid red"
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